The present invention relates to a shadow mask type color cathode ray tube, and particularly to a color cathode ray tube having a multiplicity of dot-like electron-transmissive apertures (will be also referred to as beam apertures or simply as apertures in this specification) in the useful apertured portion of the shadow mask with horizontal and vertical pitches of the apertures varied and providing a high quality display.
In general, in a shadow mask type color cathode ray tube, a multiplicity of beam apertures are formed in the apertured portion of the shadow mask in correspondence with the shape and position of the three-color phosphor screen formed on the inner surface of the faceplate of the panel portion.
The beam apertures in the shadow mask are arranged with horizontal and vertical aperture pitches determined by some formula such that triads of red, green and blue phosphor picture elements are arranged in predetermined positions.
As such a prior art shadow mask type color cathode ray tube, there are known those in which the aperture pitches of beam apertures in the apertured portion of the shadow mask are determined by the following formulas on the basis of the slightly curved shapes of the inner surface of the faceplate and the apertured portion of the shadow mask.
In one example, the horizontal aperture pitches are nearly uniform in a central portion on both sides of and in the neighborhood of the vertical center line of the apertured portion, and become progressively greater as the left or right side of the apertured portion is approached outside of the central portion. This type is hereinafter referred to as a prior art shadow mask of the first type.
In another example, as disclosed in Japanese Patent Laid-Open Publication No. SHO 56-41648, the vertical aperture pitches of the beam apertures become progressively smaller as one goes from the horizontal center line toward the top or bottom of the apertured portion and from the vertical center line toward the left or right side of the apertured portion at the same time. This type is hereinafter referred to as a prior art shadow mask of the second type.
In a color cathode ray tube employing the prior art shadow mask of the second type, the phosphor screen is configured such that the line triads of the three phosphor dots of red, green and blue colors tilt increasingly with respect to a horizontal direction as one goes from the horizontal center line toward the top or bottom of the useful phosphor screen area and from the vertical center line toward the left or right side of the useful phosphor screen area at the same time.
FIG. 5 is a plan view showing an arrangement of part of phosphor dots in the useful phosphor screen area of a color cathode ray tube employing the shadow mask of the second type (disclosed in Japanese Patent Laid-Open Publication No. SHO 56-41648).
In FIG. 5, reference numeral 31 designates a faceplate; 32 is a useful phosphor screen area; 33 is phosphor dots; 33R is red phosphor dots; 33G is green phosphor dots; 33B is blue phosphor dots; 34 is the vertical center line (Y axis) of the useful phosphor screen area 32; and 35 is the horizontal center line (X axis) of the useful phosphor screen area 32.
The faceplate 31 has a multiplicity of phosphor dots 33 in the useful phosphor screen area 32 on the inner surface thereof. The phosphor dots 33 are composed of a plurality of triads and one red phosphor dot 33R, one green phosphor dot 33G and one blue phosphor dot 33B form one triad.
As shown in FIG. 5, the phosphor dots 33 are arranged with nearly uniform horizontal and vertical pitches such that the line triads of the three-color phosphor dots 33 tilt increasingly with decreasing distance from the corners of the useful phosphor screen area 32, with respect to the horizontal center line 35.
Specifically, the line triads of the three-color phosphors are nearly horizontal in each of the following portions in the useful phosphor screen area 32: the approximately central portion and its neighborhood; the portion extending on the horizontal center line 35 from approximately the center to the right and left sides and its neighborhood; and the portion extending on the vertical center line 34 from approximately the center to the top and bottom and its neighborhood. At the four corner portions in the useful phosphor area 32, the line triads of the three-color phosphors 33 tilt most largely with respect to the horizontal center line 35 and tilt decreasingly with respect to the horizontal center line 35 with increasing distance from the four corner portions. The tilt of the line triads of the three-color phosphors 33 with respect to the horizontal center line 35 at the four corner portions have upward slopes at the upper left-hand corner, downward slopes at the upper right-hand corner, downward slopes at the lower left-hand corner and upward slopes at the lower right-hand corner. In the following description, such an arrangement of a multiplicity of the phosphor dots 33 on the faceplate 31 is referred to as a tilt array arrangement.
In the color cathode ray tubes employing the prior art shadow mask of the first type, a multiplicity of the phosphor dots formed in the useful phosphor screen area are arranged with the horizontal aperture pitch nearly uniform in the portion on both sides and in the neighborhood of the vertical center line, and with dot pitches progressively greater toward the left or right side of the useful phosphor screen area out of the central portion. Accordingly, horizontal and diagonal pitches between two adjacent triads of phosphor dots at portions at sides and corners of the useful phosphor screen area become greater, respectively. As a result, the color cathode ray tubes employing the prior art shadow mask of the first type have a disadvantage that the resolution is deteriorated at portions at sides and corners of the phosphor screen.
In the color cathode ray tubes employing the prior art shadow mask of the second type, since the phosphor dots formed in the useful phosphor screen area are arranged in a tilt array arrangement, diagonal pitches between two adjacent triads of phosphor dots at portions at corners in the useful phosphor screen area become smaller. As a result, the color cathode ray tubes employing the prior art shadow mask of the second type have a disadvantage that the color purity tolerance at each corner portion on the phosphor screen is reduced, leading to the non-uniformity in displayed colors.